Two days before the start of last year’s winter meetings, Major League Baseball’s annual convention, the Mets announced that General Manager Sandy Alderson could not attend the event in Nashville because he was dealing with a treatable form of cancer.

Everyone in baseball attends the winter meetings. All 30 teams send the members of their front offices, often accompanied by scouts and owners. Most agents are there, too.

With Alderson beginning chemotherapy, his main lieutenants agreed to turn to one point man during what turned out to be four busy and critical days in their off-season: John Ricco, the team’s assistant general manager since 2004.

“John handled it in such a way that everything that was done, I felt it was being done by Sandy, and that’s a compliment to John,” said Yankees General Manager Brian Cashman, who watched from afar.

Before those meetings, casual Mets fans knew of Ricco, if they had heard of him at all, as the team’s longtime assistant general manager. Yet inside the Mets and across baseball, Ricco was a well-known and respected executive, a steadying force inside the team’s baseball operations through years of both contending and rebuilding. He has served as the right-hand man to three Mets general managers: Jim Duquette, Omar Minaya and Alderson.

During Ricco’s time, the Mets have reached the National League Championship Series in 2006 and rebounded from financial losses sustained in the Bernard L. Madoff scandal to reach the World Series in 2015. But perhaps the best effort yet was Ricco’s role in helping the Mets’ front office patch up an injury-riddled 2016 team with in-season moves that turned them into a wild-card team.

“We’re fortunate to have him,” Alderson said. “As time has gone on, he’s taken on more responsibility, and I have tremendous faith in his judgment.”

Alderson’s health has improved since last winter, but Ricco’s function will remain the same during another important off-season for the Mets. With many injured players expected to have recovered, the Mets may be a force next season. To contend, they hope to retain the free-agent outfielder Yoenis Cespedes, an All-Star last season, but they may have to display the same nimbleness as they did last December.

With Alderson at home during the winter meetings, the Mets courted Ben Zobrist, but he signed with the Chicago Cubs. A day later, the Mets traded for Neil Walker to shore up second base and signed Asdrubal Cabrera to play shortstop. Ricco, 48, helped lead the Mets’ efforts in Nashville while staying in constant touch with Alderson, and he spoke to reporters at every turn.

“What everyone saw at the winter meetings is what we see on a day-to-day basis,” said Adam Fisher, the Mets’ director of baseball operations, who has worked with Ricco for a dozen years. “He’s calm, cool and collected in pressure situations, and he can handle himself fine as a leader. He’s been in the middle of every single one of our big acquisitions for the last 12 years.”

Ricco’s path to the Mets’ front office began in a newsroom. Growing up in Cresskill, N.J., Ricco was a Yankees fan — an allegiance that has long since fizzled. He wanted a career in sports but was not sure in exactly what capacity.

He worked at the student newspaper at Villanova, where he earned a degree in communications, and by his second semester he was the sports editor. He secured an internship at the ABC affiliate in Philadelphia and did some freelance work for television production companies.

After he graduated in 1990, Ricco sent his résumé to every television network and baseball team and received no interest. Disappointed, he cast a wider net to public relations firms, and through a chance connection he landed an interview with Jeff Idelson, then the Yankees’ director of media relations. Ricco spent 1991 as an intern in the Yankees’ media relations department.

“I was open to anything in sports, and this is kind of what happened,” he said.

That internship led to another, this time in the American League media relations office, back when each league had its own office. That led to a full-time job, and then an offer to run the league’s waiver wire. He administered all of the A.L. teams’ transactions, learning the nuances of baseball’s rules.

“I was 23 years old doing that, and that was kind of my ticket into where I am today,” Ricco said. “I started working with the G.M.s and assistant G.M.s on a daily basis and advising them on different rules.”

That sparked Ricco’s love for the details of baseball: navigating the rules, juggling the moving pieces of the major league and minor league rosters, balancing a budget.

After four years of transactions, Ricco moved to Major League Baseball’s labor relations department — a breeding ground for future team executives — and learned about the salary arbitration system and the collective bargaining agreement with the players’ union.

Over the next 12 years, Ricco moved his way up in the commissioner’s office, handling matters including player grievances, dealing with agents and helping Rob Manfred, before he became commissioner, determine the financial components of the labor agreements with the players’ union.

“I made a lot of connections,” Ricco said.

In 1998, when Cashman took over as general manager of the Yankees, he remembered the former intern who had blossomed in the commissioner’s office and tried to hire him. Ricco turned down the offer of an assistant general manager position because he felt there was more he wanted to learn abut contract negotiations and the economics of baseball.

Other teams tried to lure Ricco as well. When Minaya became the general manager of the Montreal Expos in 2002, his first call was to Ricco. Finally, in 2004, Ricco interviewed with the Mets and accepted Duquette’s offer to become his assistant general manager.

“I was brought in more because they knew I had experience with the rules and contracts,” Ricco said. “Those were the two areas and still remain my bread and butter.”

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Though unfamiliar to casual Mets fans, John Ricco has been a steadying force inside the team’s baseball operations for years.CreditSantiago Mejia/The New York Times

After a 91-loss season, Duquette was pushed aside and replaced by Minaya. The Mets made splashes over the next two years, trading for Carlos Delgado and Paul Lo Duca and signing Pedro Martinez, Carlos Beltran and Billy Wagner.

“He kept everything together when I was there and allowed me to go and pursue the talent,” Minaya said. “In a lot of ways, he was running the whole operation.”

Minaya pushed Ricco to watch games alongside the Mets’ top scouts. Ricco learned how to evaluate scouts, hire capable ones and rely on them.

Like most modern general managers, Alderson did not come from a scouting background, but he has proved to be a successful executive because of qualities that he said Ricco also possessed.

“John is a good decision-maker and takes input from a lot of different sources, and he’s able to synthesize them to make good decisions,” Alderson said. “He’s been exposed enough to scouting and player development that he’s able to manage them.”

After Minaya was fired in 2010, the Mets’ owners told Ricco they wanted to hire a general manager from outside the organization. Ricco helped line up interviews and prepared questions for the man who became his new boss. Alderson knew Ricco well from their dealings during his time in the commissioner’s office, as well as his turns as the top executive of the Oakland Athletics and the San Diego Padres.

Ricco’s duties encompass every aspect of the organization. As vice president and assistant general manager, Ricco is responsible for the day-to-day operation of an organization that has about 250 players across all levels and 175 nonplayer employees. He, Alderson and the special assistant J. P. Ricciardi are the key baseball decision-makers for the Mets.

Ricco oversees the budget. The Mets’ major league payroll was about $140 million in 2016, and their nonplayer budget was about $45 million. Ricco coordinates the medical and training staff, and keeps the owners and Manager Terry Collins abreast of players’ health.

He travels with the team on about half the road trips, giving him a chance to spend time with his wife, Blakely, and their three children when he is not on the road. He spends half his day on his email or on the phone, including time on the drive to Citi Field from his home in Cedar Grove, N.J.

“Because it’s something I love, it’s energizing,” he said. “But it is relentless. There are no two ways about it: It is 24/7.”

And because the demands are great on a general manager in the largest market in the country, Ricco has helped Alderson with the media aspect of his job. While recuperating at home during last year’s winter meetings, Alderson watched how Ricco handled himself during his televised interviews. He said he believed Ricco was ready to lead a team one day.

“He’s worked for three different general managers and learned something from every one of them,” Alderson said. “And he’s done it in a difficult market that doesn’t tolerate any mistakes too well.”

A version of this article appears in print on , on Page D1 of the New York edition with the headline: For the Mets, a Middleman on Call 24/7. Order Reprints | Today’s Paper | Subscribe